The collar counties: DuPage, Lake, Will, Kane, McHenry
Standard Illinois rules, high tax rates, and quadrennial cycles that differ by township.
The five collar counties follow standard Illinois practice — 33⅓% assessment, quadrennial reassessment, Board of Review as the first formal forum — with some of the highest effective tax rates in the country.
Why the rates are high
Heavy reliance on property tax to fund well-regarded school districts. School levies commonly make up the majority of a collar county bill, which is why two similar houses in adjacent districts can carry noticeably different bills.
What that means for appeals
A high rate multiplies the value of any assessment reduction. The same $10,000 EAV reduction is worth materially more here than in a low-rate downstate district, which changes the arithmetic on whether an appeal — or an appraisal — is worth the effort.
Data availability
The collar counties generally publish good online parcel records with characteristics and assessment history, and several publish GIS parcel layers. That makes uniformity analysis genuinely practical for an owner working alone.
Timing
Quadrennial years differ by township, not by county. Ask your township assessor which year yours is.